In blood sample testing it is well known to employ thixotropic gel-like materials in evacuated blood collection tubes to automatically obtain phase separation and a phase partition or barrier between the separated blood phases (serum or plasma and the cellular phase) during centrifugation of the blood filled tube. Such gel-like materials may include a mixture of silicone and a silicon dioxide powder such as disclosed in U.S. Pat. No. 3,852,194. In U.S. Pat. No. 4,021,340 a mixture of liquid polybutene polymer and silicon dioxide powders are used to form a partitioning material. The gel-like material is made to have a specific gravity intermediate the specific gravities of the separated phases so that the material moves during centrifugation to the phase interface to form a phase barrier between the separated phases. The phase barrier maintains the phases separated during storage or shipment, and until the low density phase, serum or plasma, is decanted or otherwise removed for clinical analysis.
In U.S. Pat. No. 4,246,123 and U.S. application, Ser. Nos. 31,817, now abandoned, filed Apr. 20, 1979, housings having standpipes or passages are used to convey the gel-like material toward the interface. In this way, the gel-like material flows from the bottom of the collection tube through the passageway and cellular phase without contacting the cellular phase. This tends to reduce hemolysis due to gel/cell collision, prevent gel material from trapping cells, and reduce the chance of trapping red blood cells in the lower density phase.
In U.S. Pat. No. 4,315,892, a valve is employed at the upper end of a standpipe in the low density phase zone of the collection tube. The standpipe is filled with gel-like material. The movable member of the valve has a specific gravity greater than the specific gravity of the low density (about 1.03) phase so that it will eventually be actuated to the open position, but is less than that of whole blood (about 1.05) so that the valve does not open immediately or too soon. When the specific gravity of the liquid decreases sufficiently below the specific gravity of the movable valve member, the valve member moves downwardly opening outlet ports in the standpipe and allowing gel-like material to flow from the standpipe to the interface of the separated phases. The valve provides a time delay permitting the barrier to be formed after phase separation has taken place and independently of the viscosity of the gel-like material and speed of the centrifuge. However, the driving force necessary to actuate the valve is relatively small since it depends on a relatively small specific gravity difference between the valve member and the liquid adjacent the member. Thus, in some cases, friction and inertia of parts can undesirably affect the operation of the valve.